Enabling Technology on Alcator CMod

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Enabling Technology on Alcator C-Mod

• Jim Irby
• MIT-PSFC

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Outline

• ICRF
• Antenna development and protection electronics
• Fast automatic and load tolerant tuning
• Lower Hybrid
• Coupler/Launcher
• Status
• Modelling
• First Wall
• Development of disruption/heat resistant hardware
• Tungsten brush prototype
• Cryopump, upper divertor, and lower outer
  divertor modifications
• High pressure puff
• Technology Collaborations

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ICRF

• Development of reliable, efficient antennas is of
  prime importance to the C-Mod Program and fusion
  research generally
• Large range of technological issues must be
  addressed and well documented
• Plasma coupling
• Real time/passive tuning
• Transmission line design
• Rf breakdown
• Fault protection/detection
• Metal/dielectric interfaces
• Impurity generation
• Mechanical integrity

4-Strap Antenna
2-Strap Antennas
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Fast Tuning System
fft
transmitter
antenna
SWRlt 1.1
Passive SWRlt1.5

• Isolate transmitter from antenna using ferrite
  tuners/stubs and combine with passive load
  tolerant components
• Expect approx 1 ms response time to changing
  plasma conditions (4 ms full range tuning)
• Tuning is currently a shot-to-shot process, so we
  expect to greatly reduce number of discharges
  required to prepare for physics runs
• Will reduce manpower requirements on the rf
  physics staff
• Protection systems even more critical since the
  tuner is capable of matching to low impedance
  arcs
• Improved phase/amplitude/fault detection
• Improved diagnostics

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Rf Breakdown Experimental Program

• Rf breakdown in antenna and transmission line
  structures, and at metal-dielectric interfaces
  limit the performance of ICRF systems
• A program to experimentally parameterize rf
  breakdown in geometries found in ICRF systems is
  being pursued
• EB --- we have found 15 kV/cm empirical limit
  --- why?
• EB
• Electric fields from 10 to 30 kV/cm
• Model systems in detail using simulation software
• Develop understanding of the physics involved

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New 4-Strap Antenna

• Installation of second LH launcher in FY2005 will
  displace important diagnostics
• We will install a new 4-Strap antenna to replace
  our two 2-Strap antennas (free up one horizontal
  port)
• Total available ICRF source power maintained
• The antenna will incorporate the experience we
  have gained with our 2- and 4-strap antennas

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Lower Hybrid Project

• In collaboration with PPPL, 3 MW of lower hybrid
  source power at 4.6 GHz is being installed on
  C-Mod
• Twelve 250 kW, CW, klystrons will be used for the
  first phase of operation 16 klystrons for 4 MW
  of source power during second phase
• 2nd launcher will be added during phase II
• Development of couplers, antennas, vacuum
  windows, phase and amplitude control systems, and
  high voltage power supplies and related
  protection systems is required

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Launcher Installation on C-Mod

• PPPL launcher scans radially
• Power splitters built into launcher

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Antenna

• 24 waveguides X 4 modules
• Each waveguide is vacuum sealed with Al2O3
  ceramic brick windows
• Windows are coated and then brazed into the
  waveguide
• Modules are vacuum sealed to the rest of the
  launcher with a 0.03 gold seal

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Lower Hybrid Status

• All klystrons have been tested
• 12 available with spares
• Phase II will require repairs and purchase of
  spare tube
• HVPS (50 kV, 208 A) is near the end of the
  commissioning phase (passed wire tests)
• Prototype launcher coupler being tested at PPPL
• Procedure for fabricating waveguide windows
  developed
• Phase/Amplitude control system well into
  development
• Klystron control and protection system nearly
  operational
• Expect first full power tests using HVPS in the
  next few weeks

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Lower Hybrid 3db LH coupler
Commercial software development has reached the
point where major LH and ICRF components can be
modelled in detail Antenna/Plasma interface?
ICRF Antenna Surface Currents
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First Wall

• Design of first wall components capable of
  withstanding large disruption forces and heat
  loads is required
• Increased triangularity and plasma shape control
• System designed for 9 T, 2.5 MA operation, 11 MJ
  / pulse
• Extensive diagnostic set for comparison with code
  results
• Rogowski eddy/halo
• Rangefinder deflection
• Thermocouples

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Tungsten Brush Divertor Module

• Development of first wall components capable of
  very high heat loads are clearly needed for
  fusion reactors and will be tested on C-Mod
• Tungsten rod PFC with copper heat sink being
  developed for burning plasma experiments
• Prototype has been tested at 20-25 MW/m2
• Collaboration with Sandia National Lab

Install vertical column of tiles in C-Mod outer
divertor
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New Cryopump and Divertor

• Cryopump
• Liquid helium core with LN2 heat shield
• Approx 20k l/s pumping speed for D2
• Upper divertor tiles extended to protect pump and
  enhance upper null operation
• Lower Outer Divertor
• Simple, easy to align cylindrical shape
• Edge effects minimized
• Reliable operation during long pulse, high heat
  load operation

High pressure Gas puffing
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Technology Collaborations

• We welcome expert participation and collaboration
  in
• ongoing C-Mod technology developments
• Design of fast ferrite and passive/load tolerant
  tuning systems
• Fault detection and related diagnostics for both
  ICRF and lower hybrid systems
• Modelling of antenna/plasma interface
• Antenna design
• RF breakdown experiments/theory
• Cryopump design
• Divertor design and materials
• And scoping studies on possible future
  initiatives
• Compact high-field side fueling
• MHD feedback/wall stabilization
• Advanced walls and liquid divertors

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21 Weeks

• Alcator C-Mod has run for 21 weeks (1995)
• Fewer engineering systems
• More people
• We are in the process of increasing personnel
  levels in several areas in part to aid in
  increased operation
• New rf engineer
• New mechanical engineer
• New rf physicist
• New technicians
• New post docs

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• During the machine inspection period we have made
  many improvements to increase machine reliability
• Cryostat seals improved
• Vacuum seals now protected from LN2
• Cooling lines replaced and re-routed
• Heater/Cooling instrumentation improved
• Improvements made to the OH-Coax connections
• TF inspection completed successfully
• RF diagnostic/fault electronics greatly improved
• Data acquisition upgrades are being implemented
• Power systems instrumentation upgraded (as part
  of long pulse operation)

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• New more robust inner divertor installed
• 8T/2MA
• Larger triangularity
• More shape control
• Upgrades to all ICRF antennas
• Mods to metal/dielectric interfaces
• Improved shielding of hardware
• Increased spacing at high voltage locations
• Extensive list of spare parts for the power
  systems has been developed

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Machine Status

• Pumpdown 5/20
• First plasma 6/07
• Tokamak Workshop 6/10-6/12 (no operation)
• Now conditioning machine for physics operation
• Developing double null high triangularity
  discharges in preparation for FIRE related
  mini-proposal and to aid in vessel cleanup ---
  exploits new divertor geometry
• New RFX DNB online with much improved diagnostics
• Rf systems coming back online
• New divertor diagnostics, fueling lines, pressure
  gauges are being brought into operation